The present invention relates to an AC to DC converter circuit that obtains DC electric power from an AC power supply while controlling an input current efficiently.
FIG. 9 is a block circuit diagram describing a conventional AC to DC converter circuit including a power factor improving circuit as described in Japanese Patent Publication No. 2005-110434 and a DC to DC converter circuit. FIG. 10 is a wave chart describing the operations of the conventional AC to DC converter circuit.
In FIG. 9, an AC power supply 1; coils 2, 26; diodes 4, 7 through 10, 15 through 18; switching devices 20, 22, 23; capacitors 30, 32; and a transformer 40 are shown. The power factor improving circuit is formed of a rectifying circuit, including diodes 15 through 18, and switching device 20. The DC to DC converter circuit is formed of a converter circuit including diodes 7, 8 and switching devices 22, 23 and a rectifying and smoothing circuit including diodes 9, 10 and capacitor 32.
As switching device 20 turns ON when the voltage Vin of AC power supply 1 is positive, a current flows from AC power supply 1 to AC power supply 1 via diode 15, coil 2, switching device 20, and diode 18, increasing the current i2 of coil 2. As switching device 20 turns OFF, a current flows from coil 2 to coil 2 via diode 4, capacitor 30, diode 18, AC power supply 1, and diode 15, decreasing the current i2. When the voltage Vin is negative, diodes 15 and 17 are electrically conductive, resulting in the same operations as described above. Therefore, by driving the switching device 20 with an appropriate gate signal, the input current is controlled to be sinusoidal at a high power factor and a DC voltage is obtained between both ends of capacitor 30.
The DC voltage obtained as described above is higher than the input voltage amplitude. Therefore, it is necessary to dispose the DC to DC converter circuit considering the occasion, in which it is necessary to obtain a voltage lower than the input voltage amplitude or the occasion, in which a small high-frequency transformer is used for the insulation from the AC input side. As switching devices 22 and 23 turn ON, the voltage of capacitor 30 is applied to transformer 40 via switching devices 22 and 23. Since a similar voltage proportional to the transformation ratio of the transformer is generated on the secondary side of transformer 40, the current i26 of coil 26 increases. As switching devices 22 and 23 turn OFF, the excitation energy stored in transformer 40 is regenerated to capacitor 30 via diodes 7 and 8, generating a reverse voltage in the transformer. The current i26 decreases while circulating from coil 26 to coil 26 via capacitor 32 and diode 10. Therefore, a desired DC voltage insulated is obtained by appropriately selecting the control signal pulse width for controlling switching devices 22 and 23 and the transformer turn ratio.
In the power factor improving circuit in FIG. 9, a current flows always through three semiconductor devices and conduction losses are caused in the semiconductor devices, resulting in large losses. In order to suppress the temperature rise caused by the generated losses, cooling means is required, and volume and costs for cooling means increases.
Therefore, it would be desirable to provide an AC to DC converter circuit that facilitates decreasing the number of semiconductor devices, through which a current flows, reducing the losses caused, improving the conversion efficiency, reducing the cooling means size, and reducing the manufacturing costs of the cooling means.
The capacitance on the output side of the DC to DC converter circuit (capacitor 32) is determined such that current ripples and voltage ripples are allowable. Therefore, the current ripples and the voltage ripples not only increase the volume and costs of the electrolytic capacitor connected to the output side but also shorten the life of the AC to DC converter circuit. The ripple component in the output voltage causes adverse effects such as malfunction or breakdown of the apparatus connected to the load.
Therefore, it would be also desirable to provide an AC to DC converter circuit that facilitates reducing the ripple current and the ripple voltage caused in the electrolytic capacitor, using a small and inexpensive electrolytic capacitor, elongating the life thereof, and providing the output voltage with an excellent quality.